There are two explanatory gaps Dr Tom McClelland University of Glasgow 1
THERE ARE TWO EXPLANATORY GAPS ABSTRACT The explanatory gap between the physical and the phenomenal is at the heart of the Problem of Consciousness. I argue that the case in favour of there being an Explanatory Gap relies on an implicit appeal to (one or both of) two deeper conceptual gaps. The first of these pertains to the subjectivity of phenomenal consciousness, which is taken to be inexplicable in objective physical terms. The second pertains to the intrinsicality of the qualities that characterise phenomenally conscious states, which is taken to be inexplicable in structural physical terms. I argue that the credibility of the leading Anti-Physicalist arguments the Conceivability Argument and the Knowledge Argument is bound to the credibility of these two gaps. Consequently, the real litmus test for any Physicalist account of consciousness is whether it can successfully address both apparent gaps. 1. THE EXPLANATORY GAP Physicalists claim that phenomenal states are nothing more than certain physical states. Anti-Physicalists hold that there is an explanatory gap between the physical and the phenomenal, and that this explanatory gap entails metaphysical distinctness. In this paper I will consider what reasons there are for thinking that the explanatory gap is genuine. I will not comment on the case for Physicalism, nor on the Anti-Physicalist s inference from an explanatory gap to metaphysical distinctness. I argue that the case in favour of the explanatory gap rests on an implicit appeal to (one or both of) a pair of deeper conceptual gaps, which I label Tivity Gap and Trinsicality Gap respectively. I will remain neutral on whether these two apparent gaps are genuine. My thesis is simply that there is a genuine explanatory gap only if at least one of these deeper gaps is genuine. I also argue that the credibility of the premiere Anti-Physicalist arguments the Conceivability Argument (CA) and the Knowledge Argument (KA) also depends on the credibility of these two gaps. I will begin by outlining how the explanatory gap is traditionally understood. I will then show how that formulation is limited, and why an appeal to the two deeper gaps is required. To advocate the explanatory gap is to hold that phenomenal consciousness is inexplicable in physical terms. I take it that a mental state is phenomenally conscious iff there is something it s like to be in that mental state for its bearer. Phenomenal qualities or qualia are all and only those features that characterise what it is like for a subject to be in a given phenomenal state. The property that every phenomenal state has, and which no non-phenomenal state has, is the property of there being something it s like to be in that state for its subject. We can call this the subjectivity of phenomenal consciousness. But there can only be something it s like to be in a mental state if there s something in particular it s like. We can call the particular way things are for the subject of a phenomenal state the qualitative character of that state. 2
What of the physical side of the explanatory gap? Defining physical is a conceptual minefield, but there are two key features to note. First, physical properties are the kind of property revealed by science. 1 Second, as a (debatable) matter of definition, physical properties are not phenomenal properties. If Physicalism is true, then phenomenal properties are ultimately nothing more than some special arrangement of non-phenomenal properties described by some ideal science. If the explanatory gap is genuine, there is no explanation of phenomenal properties in terms of physical properties. Interestingly, Levine s (1983) original account of the explanatory gap involves two claims that are, to some extent, independent. He holds that there can be no explanation of why a given physical state should constitute a conscious experience at all, nor of why it should constitute an experience that feels that specific way for its subject. In my jargon, Levine is claiming that both the subjectivity and the qualitative character of phenomenal states are inexplicable in physical terms. CA and KA each provide vivid illustrations of the explanatory gap, and exploit that gap to justify the conclusion that Physicalism is false. 2 According to CA, we can conceive of physical/functional duplicates of ourselves that differ from us phenomenally. If consciousness was explicable in physical terms, such beings would be inconceivable to us. According to KA, a subject who knows all the physical facts would not be able to deduce all the phenomenal facts. If consciousness was explicable in physical terms, such a subject would be able to infer the phenomenal truths from the physical truths. 2. SPLITTING THE EXPLANATORY GAP IN TWO There is an important challenge that might naturally be raised against the explanatory gap, and which invites parallel challenges to CA and KA. It is credible that we do not yet have an explanation of consciousness amenable to Physicalism. It may also be credible that we will never have such an explanation. But the explanatory gap makes the stronger claim that consciousness is inexplicable in physical terms; it is not merely that the physical explanation of consciousness is unavailable to beings like us, but rather that there is no such explanation. The gap would persist even for an epistemically ideal subject. What justifies this stronger claim? How do we know that the apparent explanatory gap is not merely a reflection of our (temporary or permanent) epistemic limitations? How do we know that the apparent gap would persist even for an ideal subject? Perhaps a subject with complete physico-functional knowledge and flawless inferential skills would not be able to conceive of physical duplicates that are not also phenomenal duplicates. Perhaps such a subject would be able to deduce the phenomenal facts from the physical facts. 3 The standard response to this line of thought is to argue that more of the same won t do. Any physico-functional information known by an ideal subject, and not by us, would 1 This is deliberately broad. Though more refined accounts are available, they do no accurately reflect the broad conception of physical at work in discussions of the explanatory gap. 2 For the original formulation of KA see Jackson (1982) and for a useful formulation of CA see Chalmers (2002). 3 Here I assume that such an ideal subject would have a relevantly complete conceptual repertoire. The Mary in Jackson s original thought-experiment does not meet this criterion, though Stoljar s (2005) Experienced Mary might. 3
inevitably be the wrong kind of information to explain consciousness (Chalmers, 2002 pp. 258-9). We are not epistemically ideal subjects, but even from our limited position we might have a grip on what kind of knowledge an ideal subject would have, and so on whether such knowledge could possibly entail anything phenomenal. Of course, the plausibility of this move depends upon how the notion of wrong kind is fleshed out. What is it about physical properties that guarantees no such properties could ever explain consciousness? It seems that only two serious answers have been given to the question, and in neither case has the dialectical significance of these answers properly been appreciated. Phenomenal states are essentially subjective while physical states are essentially objective. The term subjective is particularly ambiguous, but the simple definition offered earlier in the paper captures the relevant sense. A state is subjective iff there s something it s like to be in that state for its subject. A state is then objective iff there is not anything it s like to be in that state for its subject. Subjective states are those that involve an inside firstperson point of view, while objective states are those that do not. There appears to be a principled conceptual gap between the objective and the subjective. It is not merely that the objective facts with which we are familiar are unsuited to the explanation of subjective awareness. Rather, objective information is simply the wrong kind of information to explain why there is something it s like to be in a state, rather than nothing at all. Though a complete science may contain information radically different from what we find in current science, that information will still be exclusively objective (Nagel 1974, p. 527). I call this apparent conceptual chasm between the objective and the subjective the Tivity Gap. 4 By citing the Tivity Gap, one can argue that the explanatory gap will inevitably remain for an ideal subject. Where the Tivity Gap pertains to the subjectivity of phenomenal states, the next gap pertains to their qualitative character. The qualities that characterise conscious experience are intrinsic properties. But physical properties, it is claimed, are essentially extrinsic/structural properties. Physics characterises fundamental physical entities structurally. It describes the spatiotemporal structure of entities how those entities are located in spacetime. It also describes their causal properties, which are ultimately defined in terms of spaces of states that have a certain abstract structure such that the states play a certain causal role with respect to other states (Chalmers, 2002, p. 258). Physics thus describes a rich web of relations between entities, but never reveals any nonstructural properties. (Alter 2009, p. 760). Even an ideal physics will describe the world in purely structural terms. The qualities that characterise phenomenal consciousness are non-structural properties. A reddish quality stands in many relations, and has a specific location in the quality-space of all colour qualities. To describe the quality in terms of its relational profile, however, would be to miss out its essential nature its redness. If a subject experiences a reddish quality but is totally ignorant of the relational features of that quality, they would still be acquainted with what redness is. Conversely, as KA indicates, a subject with 4 Appeals to the subjectivity of consciousness and the objectivity of the physical are especially associated with Nagel (1974). Though his arguments are relevant, his sense of subjective is not in line with the sense in play here. In particular, Nagel is focused on types of viewpoint (such as the bat-ish perspective) where I am concerned with token viewpoints (such as the experiences of some particular bat). 4
complete structural knowledge would still be unable to infer what it is like to have a reddish experience. There is credibly a principled gap between the structural and the non-structural. As Chalmers argues, from structure and dynamics, one can infer only structure and dynamics. (2002, p. 259). Though physics might entail the facts of biology or of economics, these are plausibly still structural facts, just on a different scale to those described by microphysics. The principled conceptual gap between the extrinsic and the intrinsic thus indicates a principled conceptual gap between the physical and the phenomenal. I label this the Trinsicality Gap. I noted at the beginning of the paper that Levine makes two claims that can be separated. Now we can see how each claim ultimately has a different justification. The subjectivity of phenomenal states the fact that there is something it s like to be in a phenomenal state is inexplicable in objective terms. The qualitative character of phenomenal states the fact that a phenomenal state has the specific intrinsic qualities it has is inexplicable in extrinsic terms. Though the objective and structural nature of physical properties has often been cited to reinforce the explanatory gap, it has not been acknowledged that objectivity is specifically an obstacle to explaining why there is something it s like to be in a phenomenal state, while extrinsicality is specifically an obstacle to explaining what it s like to be in a phenomenal state. It is important to appreciate the dialectical significance of these two gaps. Regarding CA, what we can conceive does not show that consciousness is inexplicable in physical terms. What an ideal subject can conceive might show this, but our only insight into such a subject is provided by our appreciation of the Tivity Gap and Trinsicality Gap. Similarly, KA relies on our having some grip on what Mary s complete physical knowledge involves (Alter, 2009). The explanatory gap, and the two premiere Anti-Physicalist arguments, are credible only insofar (as at least one of) the two deeper conceptual gaps is credible. The real challenge for Physicalism is thus to overcome these apparent gaps, and we should be open to the possibility that each gap must be closed in a different way. BIBLIOGRAPHY Alter, T. (2009) Does the Ignorance Hypothesis Undermine the Conceivability and Knowledge Arguments?, Philosophy and Phenomenological Research, 79 (3), 756-765. Chalmers, D. (2002) Consciousness and its Place in Nature, in Chalmers, D. (ed.) 2002 Philosophy of Mind: Classical and Contemporary Readings, Oxford: OUP, pp. 247-272. Jackson, F. (1982) Epiphenomenal Qualia, Philosophical Quarterly, 32, 127-36. Levine, J. (1983) Materialism and Qualia: The explanatory gap, reprinted in Chalmers D. (ed.) 2002, Philosophy of Mind: Classical and Contemporary Readings, Oxford: OUP, pp. 364-361. Nagel, T. (1974) What is it like to be a bat?, Philosophical Review, 83, 435-50. Stoljar, D. (2005), Physicalism and Phenomenal Concepts, Mind & Language, 20, 469-94. 5